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cmd/compile: restore return-in-loop loopvar optimization

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but this time, correctly.
children of Returns can have For/Range loops in them,
and those must be visited.

Includes test to verify that the optimization occurs,
and also that the problematic case that broke the original
optimization is now correctly handled.

Change-Id: If5a94fd51c862d4bfb318fec78456b7b202f3fcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/472355
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
David Chase 2023-02-28 17:36:38 -05:00
parent ab28b834c4
commit 23e8f43cb1
3 changed files with 103 additions and 0 deletions
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23
src/cmd/compile/internal/loopvar/loopvar.go
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@ -50,6 +50,10 @@ func ForCapture(fn *ir.Func) []*ir.Name {
// will be transformed.
possiblyLeaked := make(map[*ir.Name]bool)
// these enable an optimization of "escape" under return statements
loopDepth := 0
returnInLoopDepth := 0
// noteMayLeak is called for candidate variables in for range/3-clause, and
// adds them (mapped to false) to possiblyLeaked.
noteMayLeak := func(x ir.Node) {
@ -95,6 +99,11 @@ func ForCapture(fn *ir.Func) []*ir.Name {
scanChildrenThenTransform = func(n ir.Node) bool {
switch x := n.(type) {
case *ir.ClosureExpr:
if returnInLoopDepth >= loopDepth {
// This expression is a child of a return, which escapes all loops above
// the return, but not those between this expression and the return.
break
}
for _, cv := range x.Func.ClosureVars {
v := cv.Canonical()
if _, ok := possiblyLeaked[v]; ok {
@ -103,6 +112,11 @@ func ForCapture(fn *ir.Func) []*ir.Name {
}
case *ir.AddrExpr:
if returnInLoopDepth >= loopDepth {
// This expression is a child of a return, which escapes all loops above
// the return, but not those between this expression and the return.
break
}
// Explicitly note address-taken so that return-statements can be excluded
y := ir.OuterValue(x.X)
if y.Op() != ir.ONAME {
@ -119,6 +133,11 @@ func ForCapture(fn *ir.Func) []*ir.Name {
}
}
case *ir.ReturnStmt:
savedRILD := returnInLoopDepth
returnInLoopDepth = loopDepth
defer func() { returnInLoopDepth = savedRILD }()
case *ir.RangeStmt:
if !(x.Def && x.DistinctVars) {
// range loop must define its iteration variables AND have distinctVars.
@ -127,7 +146,9 @@ func ForCapture(fn *ir.Func) []*ir.Name {
}
noteMayLeak(x.Key)
noteMayLeak(x.Value)
loopDepth++
ir.DoChildren(n, scanChildrenThenTransform)
loopDepth--
x.Key = maybeReplaceVar(x.Key, x)
x.Value = maybeReplaceVar(x.Value, x)
x.DistinctVars = false
@ -138,7 +159,9 @@ func ForCapture(fn *ir.Func) []*ir.Name {
break
}
forAllDefInInit(x, noteMayLeak)
loopDepth++
ir.DoChildren(n, scanChildrenThenTransform)
loopDepth--
var leaked []*ir.Name
// Collect the leaking variables for the much-more-complex transformation.
forAllDefInInit(x, func(z ir.Node) {